The present invention relates generally to optical networking, and more particularly, to a methodology of embedding network management information within individual optical transport streams in a manner enabling the management information to be read by a low-bandwidth, low-cost receiver without having to terminate or decode the full-rate payload stream.
Conventional networks for optical communication transmit information from one node to another as optical signals, but require full conversion of all data from optical form to electrical form at every node. In a ring or mesh network 100 such shown in FIG. 1a, the data may have to traverse many nodes 102 as it passes from the source 102A to the destination node 102B. Thus, the data will be converted from optical to electrical form and back many times before reaching its destination. Equipment associated with these optical-electrical-optical (OEO) conversions makes up the bulk of the capital cost of a conventional optical communication network. In addition, the electrical routing equipment used in the OEO nodes is specific to a particular modulation format and data rate, so that an upgrade to increase the capacity of a particular channel will require replacement of OEO equipment all along the route from source to destination. In the example depicted in FIG. 1b, the intermediate nodes at 102C, 102D, and 102E must all be upgraded to support the connection from node 102A to node 102B. This may incur a substantial expense in both capital and operating budgets associated with channel upgrades, and often results in delays until a complete new build can be finished.
Problems with OEO conversions are particularly pronounced in networks which use Wavelength Division Multiplexing (WDM) technology. WDM allows a single fiber to carry many distinct data channels by encoding each data stream onto its own optical wavelength, and then combining the wavelengths for transport through the fiber. Multiple fiber spans can be concatenated by inserting optical amplifiers between them. Up to 80 wavelengths can be handled in a single amplifier, so the cost per data channel can be greatly reduced through the use of WDM. However, existing WDM networks still require separation of individual wavelengths whenever OEO conversion is needed, and since each OEO converter handles only one wavelength, the cost of OEO routing nodes scales very unfavorably as traffic demands grow.
Optical communication networks with optical-optical (OO) nodes, based on all-optical routing systems which do not require conversion of signals to electronic format, have been developed to greatly reduce the initial capital cost of networks, while providing a flexible method for capacity upgrades of channels. The OO nodes traversed by the optical signal as it passes from source to destination are transparent to modulation format and data rate, so an upgrade can be achieved by changing the equipment at the source and destination nodes only. In the example of FIG. 1c, the capacity upgrade from node 102A to node 102B is achieved without changes at nodes 102C, 102D, and 102E. Thus, optically-routed networks (ORNs) are expected to yield substantial savings in both capital expense and operating expense associated with channel upgrades. The ORNs being developed today are WDM-capable: that is, the Reconfigurable Optical Add/Drop Multiplexers (ROADMs) and Photonic Cross Connects (PXCs) at the network nodes can control signal routing on a wavelength-by-wavelength basis. The route followed by a wavelength from source node to destination node through the ORN is called a lightpath. FIG. 2 shows an example of an ORN 200 in which four separate data signals are carried on the same wavelength. The solid lines show fiber routes, and the various dotted, dashed, and dot-dashed lines represent the four distinct lightpaths. Here, the terminating points are identified at multiplexer/demultiplexers 202A, 202B, 202C, 202D, 202E and 202F. The network further includes ROADMs 204A, 204B, 204C, 204D and 204E, and PXCs 206A and 206B. If there were a routing error in a ROADM 204 or PXC 206, optical spectra could not detect the fault. Thus, photonic networks need a path trace function to identify ad localize routing errors.
However, existing solutions for ORNs lack several key management functions provided by the OEO nodes in conventional networks. In particular, existing solutions for ORNs provide only very limited capability to trace a signal path through the network. Presence of a particular wavelength at a node is easily checked by optical means, but these methods cannot distinguish between different lightpaths which use the same wavelength. If mechanical failure or operator error causes incorrect routing of an optical signal, so that data is delivered to the wrong destination, the network may be unable to identify the cause and location of the fault.
One approach taken in the prior art employs overmodulating the data signal with a low-frequency tone to provide the optical path ID, as shown in FIG. 3a. Since the modulation depth of the Path ID tone must be kept small to minimize degradation of the data payload, the sensitivity of the path ID function is limited. Each lightpath present in the network is assigned a unique tone frequency, so that the lightpaths present at any point can be identified by the pattern of path ID tones present. The ubiquitous detectors for the path ID signal are wavelength-insensitive photoreceivers. These are quite cheap since they operate at low speeds, and there is no need for a tunable optical filter or optical wavelength selector. However, this method requires an extra optical modulator, which adds some cost to the system. A more serious flaw is that the tone is restricted to a very low frequency range, because of the need to avoid interference with the data modulation on the same optical carrier. FIG. 3b shows the power spectrum of the combined modulation by data and path ID tone, and illustrates the strategy of avoiding interference by setting the tone frequency below the low-frequency cutoff of the data modulation. For standard on-off-keyed amplitude modulation (OOK-AM), the low-frequency cutoff of the payload data spectrum, fL, is roughly determined by the data rate and longest sequence of zeroes or ones to be sent. For a 2.5 Gb/s signal with a maximum pattern length of 223−1 (typical of a SONET OC-48 signal), all path ID tones must be kept below a specified threshold, which leads to several problems. First, the number of distinguishable tones is limited, so it may be difficult to find a unique tone to label each lightpath in the network. Second, modulation at these very low frequencies leads to cross-gain modulation in the optical amplifiers in the network, thereby causing unintended ‘ghost’ tones to appear on other wavelengths passing through the same optical amplifier. Third, the bandwidth available for tone overmodulation decreases with lower data rates, making this approach impractical for slower SONET signals, such as OC-12 (622 Mb/s) or OC-3 (155 Mb/s).